The phenol resin is used widely in adhesives, coatings etc. because of its excellent adhesiveness. The phenol resin is compounded with various fillers and widely used in forming materials because of its heat resistance and chemical resistance. Conventionally, the phenol resin is used widely as the state-of-the-art electronics materials for electrical and electronic instruments, such as an electronic material, a semiconductor material, and a constituent material for semiconductor packages and color liquid crystals.
The phenol resin used in these materials is classically a novolac resin of phenol and formaldehyde using an acid catalyst or a resol resin using a basic catalyst, and these resins have been used for a long time. Both novolac- and resol-type alkyl phenol resins using an alkyl phenol as the starting material have also been used widely as rubber/tires for a long time.
In recent years, special phenol resins have also been developed for use in cutting-edge electronics. For example, there are phenol resins having phenol and phenol bound not via a methylene chain but via a para-alkylene chain or those having phenol and phenol bound via dicyclopentadiene etc., and these resins would also fall under the category of phenol resin in broad meaning. These resins have a molecular structure having the density of hydroxyl groups reduced by introducing a hydrophobic structure into a phenol resin, thereby aiming at improvement in hygroscopic characteristics. The former group is endowed with fluidity due to a para-alkylene structure and the latter group is endowed with, for example, an increase in Tg due to a rigid structure of dicyclopentadiene, and both the groups are used preferably as epoxy resin hardening agents for semiconductor sealing.
As phenol resins having a special structure in their main chain, those having a dibenzoxanthene structure and those having a fluorane structure have been developed. Both the dibenzoxanthene structure and fluorane structure are highly hydrophobic, rigid structures, and therefore, when these structures are used as hardening agents for epoxy resin, etc., excellent resin compositions with low hygroscopicity and high Tg can be obtained. Thus, phenol resins having a benzoxanthene derivative in their main chain have been developed. When the proportion of the dibenzoxanthene derivative is increased, the benzoxanthene derivative itself does not have a phenolic hydroxyl group and does thus not react with an epoxy group etc. and is hardly epoxylated by reaction with epichlorohydrin etc.
Up to now, catechol or resorcinol is reacted with formaldehyde etc. in the presence of an acid catalyst, to give a novolac resin (see, for example, Japanese Patent No. 3428699). According to such literatures, the hydroxyl equivalent of the resin obtained by using catechol etc. is around 60 g/eq. (hydroxyl equivalent), thus indicating that the resin does not contain a dibenzoxanthene derivative having a phenolic hydroxyl group. For this reason, it can be seen that under the generally known novolac production conditions, a dibenzoxanthene derivative structure is not formed and a dibenzoxanthene derivative having a phenolic hydroxyl group is not formed. If a dibenzoxanthene derivative structure were actually present in the resin, the novolac resin having a theoretical hydroxyl equivalent of around 60 would have a hydroxyl equivalent of 80 to 130 or so depending on the content of the dibenzoxanthene derivative.
On the other hand, a compound having a phenolic hydroxyl group in a dibenzoxanthene derivative structure has also been proposed. Although the proposed compound is not limited to the one having a dibenzoxanthene structure, such molecular structure attracts attention, probably owing to its effectiveness. A phenolic hydroxyl group is evidently possessed by the compound, but is not introduced into the main-chain skeleton of a phenol resin (see, for example, JP-A 2005-307185).